1. Field of the Invention
This invention relates to the field of signal electronics, and more particularly relates to a circuit for regeneration and bandwidth limitation of a quasi-periodic digital signal.
2. Discussion of the Background
In the field of radio, SSB (single side band) transmitters or ISB (independent side band) transmitters are used, which operate according to the process of "envelope elimination and restoration" (EER) (see, for this purpose EP-B1-0 193 655 or EP-A1-0 360 990).
In this process, first a produced SSB or ISB signal is divided into an amplitude signal and a phase signal. The two signals are then separately amplified and again combined into an HF final stage.
In this case, the phase signal is preferably a digital, quasi-periodic FM signal with an average frequency of about 200 kHz. In certain operating states of the transmitter this phase signal can temporarily fade, can be unstable in its phase, or be superposed by stray pulses, which leads to impairments of the transmission quality of the transmitter.
To avoid such impairments a circuit is necessary by which, on the one hand, the phase signal is regenerated without time delay and, on the other hand, can be limited in its bandwidth to about .+-.15 kHz.
It is now generally known to regenerate weak and disturbed signals with the help of a so-called PLL (phase locked loop) circuit. In this case, temporarily defective or undefinable (erratic) oscillations are bypassed or replaced by a voltage-controlled oscillator (VCO).
But such a solution has the drawback that the PLL circuit because of its unavoidable time constants, after each case of disturbance requires a relatively long time in each case to lock in position again. If, at the same time, a bandwidth limitation of the signal to be regenerated is necessary, the PLL circuit because of the great difference between the holding range and lock-in range and the necessary time constants can not be used at all.